This invention relates to semiconductor packaging and, particularly, to attachment of semiconductor die to substrates by solder reflow.
Flip chip attachment of a die to a substrate is a common interconnection technique, in which solder bumps attached to an active surface of the die are reflowed onto matching interconnect pads on a surface of the substrate. In a solder reflow process conventionally used to form the interconnection, the die is aligned with and placed on the substrate with the solder bumps apposed to their respective pads, and the die-substrate assembly is passed through a reflow oven which heats the assembly and then allows it to cool, to melt and then solidify the bumps onto the pads. In a conventional reflow oven, the assemblies are conveyed from one end of the oven to the other through a series of zones. Heating elements in the various zones can be independently activated and controlled to raise and then lower the temperature of the assembly as it passes on the conveyor through the oven. Typically in a solder bump reflow process, the temperature of the assembly is ramped up from an ambient room temperature by heaters in the earlier zones to near the melting point of the solder, then the temperature is raised to above the solder melting point and there held for a time sufficient to allow reflow of the bumps onto the pads, and finally the temperature is allowed to fall past the solder melting point (permitting the solder to solidify on the pads) and back down to an ambient room temperature.
Some number of interconnects made according to this conventional process include poor or marginally adequate connections, which may number in the range of a few hundred defective joints per million.
It may be possible to repair failed interconnects in defective assemblies by passing the defective assemblies a second time through a reflow oven to repeat the solder reflow process, and such an approach has been proposed. This approach is impracticable for number of reasons. Particularly, oxidation of the solder can occur following removal of the assemblies from the oven, and the oxides can interfere with subsequent formation of a good connection. Also, repetition of the reflow process significantly increases (and may at least double) the time and processing resources that must be expended to achieve attachment of the initially defective assemblies.
It is desirable therefore to improve production yield by reducing the occurrence rate of defective or marginal connections during the solder bump reflow process, without significantly increasing the budget of time and resources.